elastomeric impression materials

Question 1

two types of elastomers

Answer 1

polyesther
addition silicons

Question 2

elastic behaviour in impression materials

Answer 2

when removing an IM from contact with a tooth – after its set of course – the
IM has to stretch, flaring out at the sides to overcome the bulbous aspect of the tooth.

Ideally it recovers its original dimensions, replicating the shape of the tooth.
That’s assuming it’s perfectly ELASTIC

Question 3

elastomers formation

Answer 3

are formed by polymerization, and cross-linking of
polymer chains.
The cross-linking:-
generates ELASTIC properties
causes FLUID to SOLID transition
We also know that polymerisation MAY produce BYPRODUCTS (H2O, H2, alcohol) which
affect DIMENSIONAL STABILITY and cast compatibility.

Remember that ALGINATE has byproducts and needs to be handled carefully afterimpression taking. Is that the case for ELASTOMERS?

Question 4

impression materials 4 ideal properties

Answer 4

1. Quality of surface interaction between material & tooth/soft tissue surfaces:
   Viscosity - must flow easily
   Surface wetting - close contact with teeth/mucosa
   Contact angle - how well material envelops the hard/soft tissue surface = record fine details
2. Accuracy
   surface reproduction (ISO)
   visco-elasticity / elastic recovery
3. Dealing with removal and undercuts
   Flow under pressure (”shark fin” test)
   Tear/tensile strength
   Rigidity
4. Dimensional stability
   Setting shrinkage
   Thermal expansion/contraction
   Storage

Question 5

im viscosity

Answer 5

ability to FLOW, is vital for it to reach all the
dental tissue’s surface area.
The material has to flow readily- without too much pressure being required.

Question 6

im wetting

Answer 6

ie how closely the IM envelops the tooth surface.
The GLOBULE of IM on the RHS has a LOW contact angle, meaning a large percentage of
its volume will make contact with the target surface. That’s ideal.

HIGH CONTACT ANGLE = NOT IDEAL

Question 7

hydrophilic silicones

Answer 7

Initial addition silicones had some difficulties in making good contact with moist tooth
surfaces.
To overcome this, manufacturers introduced a non-ionic surfactant component.

Question 8

accuracy of I’m

Answer 8

surface reproduction
test = placing IM along a surface which has grooves of specified width:
20, 50 and 75um. These are pretty narrow.
To conduct the test, a uniform pressure is applied across the width of the IM.

As you can see, the 75 and 50um wide grooves have been filled by the IM.
But the 20um groove is unfilled.
We conclude that this particular IM can’t reach into such narrow niches.

Obviously an IM that records 20um grooves will give you the most accurate surfacedetail

Question 9

im is visco-elastic

Answer 9

when a load is
applied – will GRADUALLY reach the strain required.
And when the load is released, its strain level GRADUALLY drops.
Note the material does NOT return to its original dimensions.
Instead it experiences PERMANENT DEFORMATION (aka permanent strain).

Question 10

ideal IM

Answer 10

undergoes 100% elastic recovery following removal of the impression
tray - which is the same as no PERMANENT STRAIN

For a typical VISCOELASTIC IM, the ELASTIC RECOVERY is less than 100%.

Question 11

shark fin test

Answer 11

To record an undercut, the IM must first reach the extremities of what is a narrow zone, with a complex shape.
IM added to a cylindrical chamber, with a specific slot

And the IM inserted in the upper part of the cylindrical chamber – and which has to have a depth greater than that of the slot - is then forced downwards (ie pressure is applied).

Here we can see the outcome for two different materials.
The IM on the LHS flows further – so it has a greater FIN LENGTH than the other
material.
Therefore the RED IM is able to flow more under pressure, and should record DEEPER UNDERCUTS more readily.